Compared with the other distributed fiber sensors, Brillouin optical time-domain analyzer (BOTDA) sensors that based on stimulated Brillouin scattering is the sensing technology with the long distance, high measurement precision in temperature and strain sensing, which has shown tremendous application in structural health monitoring (SHM) of large-scale civil engineering, communication optical cables, oil and gas pipeline. In BOTDA systems, a continuous-wave (CW) probe signal interacts with a counter-propagating optical pump pulse through an acoustic wave in the fiber due to stimulated Brillouin scattering (SBS), leading to power transfer between the two optical signals. The CW probe light is locally amplified or reduced via SBS interaction when the frequency offset of these two optical beams is within the fiber Brillouin gain spectrum (BGS). The maximum SBS interaction occurs when the frequency difference equals to the Brillouin frequency shift (BFS) of the fiber. Exploiting the dependence of the BFS parameter on strain and temperature, BOTDA could realize accurately distributed measurements along the fiber under test (FUT).
Optical time domain reflectometer based on coherent detection (COTDR) is the technology that using coherent detection to detect the Rayleigh scattering signal. This detection method can effectively restrain the impact of spontaneous emission noise (ASE) on the power of Rayleigh scattering signal, and the COTDR is very suitable in long distance link detection such as submarine optical fiber cable that with multiple optical fiber amplifiers. COTDR is widely applied in monitoring the loss, the fault point, the connection point and the break of a extra-long distance fiber link. It is an indispensable tool for monitoring the Trans Pacific submarine cable.
Due to the nonlocal effects induced by the pump depletion and the limitation of the maximum allowed input power of the probe signal and the pump light, the performance of traditional BOTDA that using the direct detection is greatly limited. The sensing length of traditional BOTDA is less than 40 km. In addition, a fatal drawback in BOTDA is the requisite of access to both fiber-ends. This implies that the BOTDA doesn't work when a break occurs along the FUT, which extremely limited the application field of the BOTDA. However, with the development of the electric power transmission network, the increasing in the structural health monitoring (SHM) of large-scale civil engineering and the demand in major disaster prevention and control, higher performance of long distance distributed temperature and strain sensing network is required.